In woodworking applications, it is often necessary to bond wood pieces together to produce a wood product. For example, recent environmental regulations and depletion of old-growth timber supplies have made it increasingly difficult and expensive for manufacturers to obtain high-grade lumber to use in wood products, which may include furniture, cabinets and millwork. One way of addressing the shortage and high cost of high quality lumber is the use of veneered stock. Typically, veneered wood products are created by laminating high quality veneer over a lower grade core material, such as medium density fiberboard (MDF), particle board, plywood or finger-jointed stock. Use of veneered wood members results in a substantially more efficient utilization of high quality wood, and therefore reduces raw material costs. Other examples of wood products produced by joining wood members together include door and window jambs, plywood, laminated veneer lumber, and other laminated wood products.
Wood used in construction varies significantly in moisture content. An elevated moisture content of wood members may add time and cost to a lamination process. Recently-harvested timber may have a moisture content of 50–60% or higher, which may be further increased by transport of the timber along a waterway, or storage of processed lumber in a wet environment. Prior lamination methods frequently require a kiln-drying step prior to bonding to remove excess water, even from green dimensional lumber which typically has a moisture content of about 18%. After kiln drying, wood has a moisture content of about 12%, providing a relatively reproducible surface environment for bonding. However, the drying step prior to lamination introduces a significant additional cost and increases the processing time.
For most woodworking applications, a wood bond must be strong in order to provide structural strength and stability. For example, it is often desirable to form a “high-strength wood bond” that has a shear-strength exceeding the shear-strength of the wood itself. Generally, high strength wood bonding procedures require application of an adhesive to a wood surface, and subsequent pressing of the wood surface against another wood surface or against a polymeric material such as PVC, polyethylene, polystyrene, polypropylene, phenolic paper and wood fiber composites with any one of the above-listed polymers.
One significant limitation with prior wood bonding techniques is that the procedure required to produce a high-strength wood bond may take a long time, for example, several hours, to produce a cured product.
Other procedures can be performed more rapidly by using an adhesive that is activated to some extent during the pressing process. For example, adhesives may be activated by applying heat. These adhesives are referred to as “thermoset adhesives.”
Presses may use heating platens or radio frequency mechanisms to activate and speed up significantly the cure time of a thermoset adhesive. Presses with heat activation mechanisms are somewhat complex, and expensive. These presses may also be limited in their ability to achieve uniform curing in some composite configurations.
Another way of activating an adhesive during pressing is to use a two-part adhesive system in which the two parts are substantially separate and unmixed until the pressing step, sometimes referred to as a “honeymooning” process. For example, see U.S. Pat. Nos. 5,944,938 and 5,626,705. However, a problem with this approach is that the pressing step may not adequately or reproducibly mix the two adhesive parts, thereby creating an inferior or inconsistent bond, or causing delays in the curing process. Another problem with a honeymooning process is that it is difficult to control the actual ratio of mixed adhesive components due to variable penetration or dilution of the components into the wood prior to mixing, particularly where the moisture content of the wood is variable.
There is a need for universal simplified wood bonding systems and procedures that can produce a rapidly-curing, high-strength wood bond between different types of wood pieces having a wide range of possible moisture contents, and between wood and polymeric materials such as vinyl, without requiring complicated adhesive activation steps while the wood pieces are being pressed.